Lubricating oil compositions for use in crankcase engine oils comprise a major amount of base stock oil and minor amounts of additives that improve the performance and increase the useful life of the lubricant. Crankcase lubricating oil compositions conventionally contain polymeric components that are used to improve the viscometric performance of the engine oil, i.e., to provide multigrade oils such as SAE 5W-30, 10W-30 and 10W-40. These viscosity performance enhancers, commonly referred to as viscosity index (VI) improvers, include olefin copolymers, polymethacrylates, styrene/hydrogenated diene block and star copolymers and hydrogenated isoprene linear and star polymers.
Olefin copolymers (or OCP) used as VI improvers conventionally comprise copolymers of ethylene, propylene and, optionally, a diene. High ethylene content OCP VI improvers are known to provide reduced lubricating oil resistance to cold engine starting (as measured by “CCS” performance). However, polymer chains having long ethylene sequences have a more crystalline polymer structure. Crystalline polymers have been found, primarily at low temperatures, to interact with waxes in the oil and other OCP chains, which results in uncontrollable increases in low temperature viscosity and, in extreme cases, the gelling of the lubricating oil. These problems have been found to manifest in Ziegler Natta polymerized OCPs containing greater than about 60 mass % polymer derived from ethylene (hereinafter referred to as “high ethylene content”, or “crystalline” OCP(s)”).
VI improvers are commonly provided to lubricating oil blenders as a concentrate in which the VI improver polymer is diluted in oil to allow, inter alia, for more facile dissolution of the VI improver in the base stock oil. A typical VI improver concentrate can contain as little as 4 mass % active polymer, with the remainder being diluent oil. A typical formulated multigrade crankcase lubricating oil may, depending on the thickening efficiency (TE) of the polymer, require as much as 3 mass % of active VI improver polymer. An additive concentrate providing this amount of polymer can introduce as much as 15 mass %, based on the total mass of the finished lubricant, of diluent oil.
There has been a continued demand for lubricating oil compositions providing improved fuel economy. There has also been a continuous demand for VI improvers that provide improved CCS performance in formulated lubricating oil compositions, without wax and polymer chain interaction (gelling). Much effort has been made in these respects to select the proper base stock oil and to provide a low ethylene content (amorphous) VI improver having improved CCS performance. However, little attention has been paid to the selection of the diluent oil used to form the VI improver concentrate. As the additive industry is highly competitive from a pricing standpoint, and diluent oil represents one of the largest raw material costs to the additive manufacturers, VI improver concentrates have commonly contained the least expensive oil capable of providing suitable handling characteristics; usually a solvent neutral (SN) 100 or SN150 Group 1 oil. Using such conventional VI improver concentrates, the finished lubricant formulator has needed to add a quantity of relatively high quality base stock oil, as a correction fluid, to insure the formulation CCS remained within specification.
As lubricating oil performance standards have become more stringent, there has been a continuing need to identify components capable of conveniently and cost effectively improving overall lubricant performance. Therefore, it would be advantageous to be able to provide a VI improver concentrate that delivers improved cold temperature performance, regardless of the VI improver employed, without requiring use of correction fluids.